Lorenza Petrini: “Modeling functional fatigue and plasticity of Shape Memory Alloys: applications in the biomedical field”

Shape Memory Alloys, and in particular Nickel-Titanium alloys, are commonly adopted for producing minimally invasive devices as self-expandable stents, aortic valves, stent-grafts (exploiting pseudo-elasticity) and microactuators, micro-pumps, surgical instrumentation (exploiting shape memory effect). These devices are subjected to cyclic loadings (due to blood pulsatility or leg movements or repeating procedures), that can induce fatigue fracture, and also may be subjected to very large deformations (due to crimping procedure, or physiological tortuous path, or overloads), that can induce material yielding. Accordingly, for a correct numerical prediction of their performance, models including progressive strain accumulation during repeated thermo-mechanical cycles (called functional fatigue) and plasticity effects on phase transformation are necessary.
In this presentation, a constitutive model, able to consider both functional fatigue and plasticity is proposed; its efficacy is evaluated comparing experimental tests and numerical predictions. The model, implemented in the commercial code ABAQUS through the UMAT user subroutine, is used to evaluate the influence of functional fatigue and plasticity on the response of biomedical devices subjected to boundary conditions resembling the implanting procedure and in-vivo loads.

For further information:
federica.migliaccio@polimi.it
roberto.fedele@polimi.it